Free piston engine control



Feb. 18, 1958 D. M. DILD'INE 2,823,653

FREE PISTON ENGINE CONTROL Filed June 26,` 1956 2 Sheets-Sheet 1 Arm/wmf Feb. 18, D, M, DILDINE 2,823,653

FREE PISTON ENGINE CONTROL Filed June 26. 1956 2 Sheets-Sheet 2 tmf IN V EN TOR.

ATTO/wr United States Patent-O FREE PISTON ENGINE CONTROL Dean M. Dildine, La Grange, Ill., assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware Application June 26, 1956, Serial No. 594,006

7 Claims. (Cl. 123-46) This invention relates to free piston engines and more particularly to a control for a free piston engine of the gasier type A typical gasier has a power piston that is movable in a power cylinder and a compressor piston that is connected to the power piston and movable in a compressor cylinder. One end of the compressor cylinder is closed to form a cushion chamber on one side of the compressor piston .and the other end of the compressor cylinder is valved so that the other side of the compressor piston can pump air to a compressed air reservoir which surrounds the power cylinder.

The power portion of the gasiiier works on the twostroke diesel cycle. The energy that is produced by combustion of fuel in the power cylinder produces outward strokes of the piston assembly and inward strokes are produced by energy which is stored up in the cushion chamber by the outward strokes. The compressor portion of the gasier delivers compressed air to the air reservoir on the inward strokes tand the reservoir feeds the power cylinder on the outward strokes. The power cylinder exhausts on the outward strokes to a gas turbine that. can furnish shaft horsepower to a suitable load as, for example, to an electric generator. The gasiiier exhaust comprises a high pressure, high tempenature mixture of incompletely expanded combustion gases and scavenging air.

When a free piston gasier is operating at a given exhaust pressure, it is necessary that sufficient energy be stored in the cushion chamber on an outward stroke of the piston to return it to the same inner dead point (I. D. P.) that it had on its previous stroke and thereby obtain the compression in the power cylinder that is required to sustain the operation of the gasifier at the given exhaust pressure. The exhaust pressure must, of course, rise and fall with the load on the turbine and at a. higher load -it is necessary to store more energy in the cushion chamber to sustain operation at the higher exhaust pressure. This is accomplished by increasing the stroke of the piston and by increasing the mass of air in the cushion chamber.

An object of the invention is to provide a control for obtaining the proper stroke and the proper mass of air in the cushion chamber for the correct operation of a free piston engine at changing exhaust pressures.

The invention provides a control for transferring air from the air reservoir to the cushion chamber when the exhaust or discharge pressure is increasing and for transferring air from the cushion chamber to the air reservoir when the exhaust pressure is decreasing so that the stroke of the piston and the mass of air in the cushion chamber are correctly related to the changing exhaust pressures.

Further objects and advantages of the present invention will -be apparent from the following description, reference being had to the accompanying drawings, wherein a preferred form of the present invention is clearly shown. Y

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In the drawings:

Figure 1 is a schematic representation, partially in section, of a free piston power plant incorporating the invention;

Figure 2 is a pressure-stroke diagram illustrating various stroke and pressure conditions of the gasier at different exhaust pressures; and

Figure 3 is a plan view, partially in section, of the inventive control.

Referring now to the drawings, a conventional free piston power plant is shown in Figure 1. The power plant consists of a free piston gasiiier 10 which exhausts by way of a conduit 12 to a gas turbine 14 that drives a generator 16 through 1a shaft 18. A variable load vis applied to the generator terminals 20 and this necessitates a variance in the gas energy that is delivered to the turbine 14. The variance in gas energy is primarily accomplished yby raising and lowering the exhaust pressure of the gasitier since the exhaust temperature of a free piston engine is not subject to wide variation. The gas turbine 14 has a governor 22 that regulates a gasiier fuel rack 24 in accorda-nce with the demand on the turbine.

The gasier has a central power cylinder 26 that has intake ports 28 and exhaust ports 30 and which slidably receives a pair of opposed power pistons 32 and 34. A pair of compressor pistons 36 and 38 lare rigidly secured to the respective power pistons and are slidably received in compressor cylinders 40 and 42. An engine case 44 encircles the power cylinder 26 in spaced relation thereto and a set of valve plates 46 and 48 close olf the outboard ends of the case 44 to form an annular lair reservoir S0. A pair of end covers 52 and 54 close off the outboard ends of the compressor cylinders to provide cushion chambers 56 and 58.

A fuel injection pump 60 supplies measured amounts of fuel in accordance with the position of the fuel rack 24 to a fuel injector nozzle' 62; the fuel injection is in timed relation with the strokes of the pistons and the fuel combusts in combustion chamber 64 on the inward strokes. Suction valves 66 and 68 deliver atmospheric air to compressor chambers 70 and 72 on outward strokes and delivery valves 74 and 76 transfer compressed air to the air reservoir 50 on inward strokes. A balance tube 78 places the cushion chambers 56 land 58 in communication with each other to keep the pistons pneumatically phased, and mechanical phasing is -achieved by synchronizing racks 80 and 82 that are secured to the respective pistons and that are geared together by a pinion 84.

The arrangement thus far described is conventional and its operation is well understood. A complete cycle of operation consists of the piston assemblies moving from O. D. P. to I. D. P. to O. D. P. The inward stroke of the pistons sends compressed air to the air reservoir and compresses air in the combustion chamber, the energy for the inward stroke being supplied by expansion of the trapped compressed air in the cushion chambers. Fuel is injected when the pistons approach I. D. P. or inner dead position to power them on the outward stroke. The outward stroke lls the compressor chambers with atmospheric air and increases the pressure of the trapped air in the cushion chambers. The power cylinder intake and exhaust ports open at the outer dead position or O. D. P. to scavenge the combustion chamber with compressed air from the air reservoir and to deliver the high temperature, high pressure exhaust gases to the turbine.

On viewing the arrangement thus far described, it is apparent that :an increase in load on the generator 16 must be accompanied by an increase in the Amass ow ofthe exhaust gas to the turbine 14. The governor 22 shifts the fuel rack 24 to provide additional fuel and increase the amount of exhaust gas flow. Since a free piston engine inherently operates at substantially constant speed, the increase in gas ow is achieved by lengthening the stroke of the pistons and by thus raising the pressure in the exhaust conduit 12. The O. D. P. of the pistons will move outward if the combustion energy is increased and the I. D. P. of the pistons will move inward because ofthe additional energy that will be stored in the cushion chambers This shift in I. D. P. and O. D. P. must be controlled to maintain optimum combustion pressures from -no-load to full-load and the invention provides a control 86 to accomplish this. l

' As previously pointed out, if the free piston gasierl is operating at a given exhaust pressure the cushion charnbers must store suicient pressure energy t return the pistons to approximately the same I. D. P. they had .on the previous stroke to sustain balanced operation at the given exhaust pressure. At a higher exhaust pressure a greater quantity of energy Imust be stored in the cushion chamber and this can be accomplished by increasing the O. D. P. and -by increasing the mass of air in the cushion chamber. The control device 86 provides a means of transferring air from the air reservoir 50 (the air pressure in the air reservoir being relatively unaffected by the movement of the piston assemblies and being directly related to the exhaust pressure) to the cushion chambers 56 and 58 (the air pressure in the cushion chambers rising and falling with the outward and inward strokes of the piston assemblies) when the exhaust pressure increases or from the cushion chambers to the air reservoir when the exhaust pressure decreases. This transfer of air tends to keep the I. D. P. from changing substantially and thisy is desirable as a wide I. D. P. range means that the compressor will have poor volumetric eiciency.

I Referring to Figure 3, the control device 86 has a rocking shaft 88 which is oscillated with each stroke by the synchronizer racks 80 and 82 through a bevel gear 90 which engages a bevel gear 92 that is secured to the synchronizer pinion 84. The rocking shaft 88 carries a cam lobe 94 which receives a cam follower 96 that is pivoted at 98 to a floating lever 100. One end of the floating lever 100 is pivoted at 102 to a piston valve 104 that is reciprocal in a valve cylinder 106 that has an interior communication with a conduit 108 that leads to the K ing lever 100 has a pivot pin 115 which carries a roller 116 which is pressed by a tension spring 118 against the surface of a cam 120, the cam having a fixed pivot at 122.

Oscillation of the rocking shaft 88 causes the cam follower 96 to move up and down and the oating lever 100 accordingly swings through an arc about the line of contact 124 of the roller 116 and cam 120. The swinging of the floating lever 100 reciprocates the sleeve valve 104 causing the ports 112 and 114 to register at some point in the stroke of the piston valve and at some corresponding point in the stroke of the free piston assemblies. When the ports register, compressed air is free to move from the cushion chambers to the air reservoir or vice versa depending upon which pressure is the highest at thatpoint in the stroke. As long as the line of contact 124 of the roller 116 and cam 120 does not shift the ports 112 and 114 will always meet at the same point in the strokes of the free piston assemblies. It is desirable to `change the relationship of port registry to stroke when the 4 exhaust pressure changes and this is accomplished by moving the cam and hence the location of the line of contact 124.

The cam 120 carries an arm 126 having a pivot pin 128 that is slidable in a slot in a reciprocal rod 130. The rod 130 carries a piston 132. that is received in a cylinder 134 which is connected by a conduit 136 to any pressure that reects the changes in turbine load, this pressure being hereafter referred to as the control pressure. The control pressure acts on the underside of the piston 132 in opposition to a compression spring 138 so that the cam 120 will be rotated in accordance with the changes in turbine load. The upper side of the piston 132 can be vented to atmosphere, as shown, or can be loaded by a different control pressure. The control pressure in the illustrated embodiment of the invention is the air reservoir pressure which is slightly more than, but

which follows, the exhaust pressure. Other pressures that indicate turbine load can be used, for example, a control pressure generated by the turbine governor, the gasier exhaust pressure, or a control pressure generated by a torquemeter on the turbine shaft.

The arrangement is such as fo cause the air reservoir and the cushion chambers to be placed in communication with each other at increasingly closer points with respect to the center of the engine, i. e., the mechanical I` D. P., and at increasingly distant points with respect to the ends of the engine, i. e., the mechanical O. D. P., as the exhaust pressure increases with increased load. An increase in the air reservoir pressure, as results from increased load, raises the control piston 132 and causes right hand rotation of the control cam 120 to raise the left hand end of the floating lever'100. Raising the left hand end of the floating lever 100 drops the right hand end since the center of the lever is restrained by pivot 98 and thereby moves the ports 112 and 114 nearer to registry. The upward stroke of the piston valve 104 corresponds to the inward stroke of the free piston assemblies (the I. D. P. of the piston valve is at the top of cylinder 106 in Fig. 3) and the timing of the communication between the cushion chambers and air reservoir with respect to the free piston stroke position is accordingly modified, i. e., the valve ports register nearer to I. D. P. on increases in exhaust pressure.

Figure 2 is a pressure-stroke diagram illustrating the action of the control 86 in obtaining the correct stroke and the correct mass of air in the cushion chambers for the most efficient operation of the gasier at various discharge pressures. If the gasier is operating with a mean exhaust pressure 1 it will have an air reservoir or case pressure 1A and a cushion pressure 1B. The exhaust pressure uctuates somewhat, depending on the capacity of the conduit 12, and the diagram shows mean exhaust pressure as isv substantially obtained with a very large capacity conduit or a conduit-accumulator combination. If thev load is increased and the exhaust pressure accordingly raised to 2, the gasiter will have a new case pressure 2A and a new cushion pressure 2B. A further increase in exhaust pressure to 3 will result in a raised case pressure 3A and a raised cushion pressure 3B. The valve open area A denotes the portion of the stroke that the ports 112 and 114 are in registry. The horizontal width of the area A represents by distance and location where the cushion chamber and air reservoir are in communication with each other on each stroke. The shape of the area A and its location on the pressure-stroke diagram is determined by the shape of the cam 120. If the left end of the floating lever 100 did not move with changes in turbine load, the valve open area A would be a wide vertical line and communication between the air reservoir and cushion chambers would not take place at the proper point in the stroke of the pistons. During stable load conditions there is little or no change in exhaust pressure and the ow between the air reservoir and cushion chambers is very small and is predominantly from thereservoir to the cushions to make up for air leakage from the cushions past the compressor pistons.

While the preferred embodiment of the invention has been described fully in order to explain the principles of the invention, it is to be understood that modifications of structure may be made by the exercise of skill in the art within the scope of the invention which is not to be regarded as limited by the detailed description of the preferred embodiment.

I claim:

l. A control for a free piston engine having a free piston, a cushion chamber and an air reservoir, the control comprising means synchronized with the free piston for placing the cushion chamber and the air reservoir in communication with each other over some portion of each stroke of the free piston and means for shifting the relative location on the stroke of the portion of communication toward the mechanical I. D. P. of the free piston in accordance with increases in the load on the engine.

2. A control for a free piston engine having a free piston, a cushion chamber and an air reservoir, the control comprising a valve synchronized with the free piston including a pair of members that are movable with respect to each other on each stroke of the free piston, the members having ports that register with each other at some point on each stroke of the free piston to place the cushion chamber and the air reservoir in communication, and means for shifting the relation of the members in accordance with operational changes in the engine so that the ports register at other points on each stroke of the free piston.

3. In combination with a free piston engine having an engine cylinder, a free piston in the cylinder and an air cushion for moving the free piston on the compression stroke, a control comprising means synchronized with the free piston for adjusting the air cushion pressure over some portion of every stroke of the free piston and means for shifting the relative location on the stroke of the portion of adjustment in response to changes in the operating characteristics of the engine.

4. Apparatus in accordance with claim 3 wherein the relative location on the stroke of the portion of adjustment becomes increasingly distant from the mechanical O. D. P. of the engine in response to increases of load on the engine.

5. In a free piston gasiier of the type comprising a power cylinder, a compressor cylinder, an air reservoir, a pneumatic cushion, a free piston in the cylinders, the compressor cylinder being arranged to deliver air to the air reservoir, the air reservoir being arranged to scavenge the power cylinder, the exhaust gas from the power cylinder constituting the motive fluid generated by the gasitier and the pneumatic cushion being arranged to receive energy from the free piston during the expansion stroke in the power cylinder and to return energy to the free piston during the compression stroke in the power cylinder, in combination, a cushion control operable while the gasiier is running for varying the mass of gas in the pneumatic cushion comprising means responsive to changes in the exhaust pressure of the gasifier, a valve for connecting the pneumatic cushion with the air reservoir, and means connected to the free piston and to the valve to open the valve over some portion of every stroke of the lfree piston and connected to the first mentioned means to vary the relative location on the stroke of the portion of opening of the valve in accordance with the pressure load on the gasiiier.

6. In a free piston gasiier of the type comprising a power cylinder, a compressor cylinder, an air reservoir, ra pneumatic cushion, a free piston in the cylinders, the compressor cylinder being arranged to deliver air to the air reservoir, the air reservoir being arranged to scavenge the power cylinder, the exhaust gas from the power cylinder constituting the motive tluid generated by the gasier and the pneumatic cushion being arranged to receive energy from the free piston during the expansion stroke in the power cylinder and to return energy to the free piston during the compression stroke in the power cylinder, in combination, a cushion control operable while the gasiier is running for varying the mass of gas in the pneumatic cushion comprising a cam movable with changes in load on the gasitier, a member movable with each stroke of the free piston, a valve movable to connect the pneumatic cushion with the air reservoir, and a iloating lever connected to the valve, to the member and to the cam to open the valve on each stroke of the free piston at a relative location on the stroke determined by the load on the gasier.

7. Apparatus in accordance with claim 6 wherein the valve opens at relative locations on the stroke that are increasingly -distant from the mechanical O. D. P. of the free piston as the load on the gasilier increases.

References Cited in the tile of this patent FOREIGN PATENTS 556.454 Great Britain Oct. 6, 1943 579,733 Great Britain Aug. 14, 1946 579,734 Great Britain Aug. 14, 1946 

